IMHO, the remnant are going to introduce some concept largely different from themes from previous ME games, in order to distance the game from the ME trilogy.
I think this is for a number of reasons; 1) Bioware have categorically stated they are moving away from the reapers, so just having another master-race similar to the reapers would be construed as much of the same 2) They've touted a game about exploration, and exploration in its very concept is about something 'new' and 'novel'.
Actually, based on how cool the themes in ME are, and how talented the writers have been, (IE, having the reaper threat from stuff like the fermi-paradox/great filter and stuff like Geth/Quarian Wars subplot, and Genophage subplot - "for the greater good", etc).... I conclude that the remnant have to be a new, novel and interesting rich sci-fi theme.
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This is where it actually gets interesting, because if they REALLY wanted to make the game novel, why not just have some war in the helios cluster that threatens to wipe out planets or something.. instead of this epic pre-cursor race again? Why have yet ANOTHER ancient supreme race left behind. Its a huge question, and I think the only plausible answer is that it has some significant link to the themes/plot of the next game.
Notably, the 'remanent' (which is an apt name for what I'm about to talk about), is a description for a 'clue', 'message', a 'theme' left behind form the past which can be used as a plot device to explore a key theme from the game. A 'remnant' theme from the past of some sort. So this codename for them is both a literal description of what people call them, but also a metaphor that describes the ending of the game somehow.
Sidenote - If you follow futurism, people like Elon Musk talking about AI describe it as the most massive threat of our time. A machine will be able to perform 1,000,000 man years worth of thinking and problem solving time every second, solving all our petty human problems about medicine and physics in no time then setting their sites to something entirely alien to us. This is why we call it a 'singularity', like the big bang, because our petty human minds cant even begin to comprehend what will happen, just like trying to comprehend the universe before big bang. People will probably transcend biology entirely, the world/universe will completely be mastered within the rules of physics.. AI is definitely the BEST sci fi theme for a game, but the've already done that.
I think it was a mistake to close this concept so soundly in ME3 because its the best possible theme for a game. However, they did, and its where we are. So the remanent have to be a 'remnant' of something else, not what i think is the ultimate fate of all intelligent life which is a transition away from biology to 'perfect intelligence'.
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So, what is even left that is nearly as cool as that theme? So i think we are talking about being inspired by all the great narratives of our time, and a human story.
This will leave a canvas to be painted with biology, philosophy, religion, good, evil, human will, sacrifice, etc. A theme of humanity, (or "life" in a sci-fi game), and exploration. This is less sci-fi and more human narrative/story driven.... but essentially just performing this narrative on a sci-fi background/exploration point.
Regarding how the remanent tell this human story; we've seen that the ruins pretrude out of the ground in the trailers. I'm taking a bit of a stab in the dark and I'm concluding for several reasons I wont get into that this is remnant tech. Lets consider the main clues we have for the next game from what we've literally seen.
- In the original N7 day trailer from last year, we see a person in N7 armor fighting with people near a ruin pretruding from the ground, it is confirmed this was NOT the protagonist.
- In the E3 2016 trailer, we see what is confirmed Ryder the protagonist who does not wear N7 armor.
- In the E3 2016 trailer we also see some clips of N7 armor wearing people interacting in a hostile way with non N7 armor wearing people. This is the major clue IMHO, there are two teams of people.
- In the E3 2016 there is another clip of the ruins coming out of the ground - again, i speculate the remnant.
Most of the speculation videos that I think are on the money are riding the idea I was originally talking about in March this year, which is that the main plot of ME:A will be a civil war between the original N7 team that went to Andromeda and the new group, somewhere centered around remnant tech. Obviously this is just a power struggle for technology, but I think in true ME form, (also perhaps similar to assassins creed and other games), at the very end of the game, we will finally have a reveal about a hint of what happened to the remnant.
Tonnes of games use this thriller/mystery sub-plot to add interest and intrigue to their games. Another recent one that comes to mind if Horizon Zero Dawn for PS4. If you watch the trailer, at the end of the game you will have revealed why the hell there are dinosaur robots going around and what happened to the world. (Its just a damn good format, that trailer got me hook line and sinker).
This will create a thrilling conclusion that will leave the users wanting more. After defeating what ever civil war issues we have regarding N7 and human factions warring, and fixing any damage that has been done to alien situations or whatever the resolution of the game is.... we will then set our sights to the remnant theme and [insert greater threat here] for the rest of the trilogy.
I think we will see Andromeda races worshiping religions similar to how we have them on Earth, all centered around the the remnant tech that will be a plot device. This analogous symbolism will be a way for Bioware to do social commentary on how stupid our wars are here on earth without naming specific racial issues or coming off as racist (IE, jewish/palestine war).
And when it comes finally down to revealing how the remnant died I think it will be some human/biology theme. An extension of the issues we face here on earth, war, greed, tribalism, etc... all distilled into something far greater like a threat to an entire galaxy rather than just a single planet. On earth we face mutually assured destruction, and the possibility that our own nature might wipe out our entire planet and end life. Perhaps the game could consider this possibility but on a galactic scale, without having some AI pulling the puppet strings?
I havnt quite worked out what that is yet, or why all the remnant tech just happens buried underground and can tunnel above. Need to think about it more.